Lithium ion secondary batteries are known as secondary batteries that have high charge-discharge capacity and that are capable of high output. Currently, lithium ion secondary batteries are used primarily as power sources for portable electronic devices, and are also promising as power sources for electric vehicles that are anticipated to become more prevalent in the future.
Lithium ion secondary batteries have, at the positive electrode and negative electrode, respective active materials capable of intercalating and desorbing lithium (Li), and they operate by migration of lithium ion in the electrolyte solution between the two electrodes. In a lithium ion secondary battery, the positive electrode active material used is mainly a lithium-containing metal complex oxide such as lithium cobalt complex oxide, while the negative electrode active material used is mainly a carbon material with a multilayer structure.
However, the capacities of existing lithium ion secondary batteries cannot be considered satisfactory, and higher capacities are desired.
In this regard, it has been proposed in recent years to raise the maximum operating potential of the positive electrode, thereby increasing the open-circuit voltage between the battery terminals. Specifically, the post-charging open-circuit voltage between battery terminals in a common lithium secondary battery of the prior art, has been 4.2 V or below upon completion of charging, and it has therefore been proposed to further increase the voltage in order to achieve even higher capacity for the lithium secondary battery. Yet increasing the voltage exacerbates the problem of oxidative decomposition of the electrolyte solution near the positive electrode during charging.
In response to this problem, Patent Document 1 teaches that, during formation of a positive electrode active material layer for a lithium ion battery that operates at high voltage, it is possible to improve the durability of the obtained lithium ion battery by using an inorganic phosphate such as lithium phosphate in addition to the positive electrode active material.